A typical conventional circuit for generating a surround-sound (hereinafter referred to as surround-sound circuit) includes a RAM 11 (hereinafter referred to as delay RAM) and a digital signal processing circuit 12, as shown in FIG. 1. The delay RAM 11 and the digital signal processing circuit 12 are provided to generate a surround-sound as follows.
The audio input signal IN input to the digital signal processing circuit 12 is first written in the delay RAM 11, as seen in FIG. 1, for holding the data for a predetermined period of time, from which the signal is read out at the end of the period and added to the current input signal IN as a surround-sound signal.
Such a conventional surround-sound circuit as shown in FIG. 1 has an obvious drawback that the audio input signal IN is directly written in the delay RAM 11, so that the delay RAM 11 must have a disadvantageously large storage capacity.
FIG. 2 shows another conventional surround-sound circuit which includes a down-sampling converter 23 for lopping off part of the audio input signal IN to reduce the sampling frequency to one half of the original frequency, for example, of the audio input signal IN before writing the audio input signal IN in the delay RAM 21. The surround-sound circuit is also provided with an over-sampling converter 24 which steps up the sampling frequency of the down-sampled data when the down-sampled data is retrieved from the delay RAM 21 a predetermined time later. The over-sampling is made by interpolating the down-sampled data. The interpolated data is then added to the current audio input signal IN as a surround-sound signal. This approach is useful in reducing the required storage capacity of the delay RAM 21. However, the surround-sound is much degraded compared with the original audio input signal due to the down-sampling and the over-sampling.
Thus, conventional surround-sound circuits have disadvantages in that they require a large memory to store the audio input signals IN or they must tolerate degradation of the surround-sound caused by lopping of data.
It is therefore an object of the invention to provide a method and an apparatus for generating a high-quality surround-sound signal with a greatly reduced storage capacity RAM, without lopping the audio input signal.